In the use of apparatuses, e.g. heat exchangers, valves, pumps, mixers or drying apparatuses for processing liquid substances, it is of utmost importance for optimum function of the apparatuses that the liquids are totally separated from the external parts of the apparatuses, so that contaminants are not transferred from the product side to the external side and vice versa, which might cause cross-contamination of the liquid substances and the external parts of the apparatus. It is likewise of utmost importance that the product side is completely purified at the beginning of the production, so that a product contacting the surfaces is not contaminated by contaminants from the preceding production.
Contact between the liquids at the product side and external side is mainly caused by leaks—holes, cracks and the like in the material separating the internal and the external side of the apparatuses. By way of example in a heat exchanger, it will be readily understood the internal side may be the product side and the external side may be the service medium compartment. In other words the exterior of the apparatus can also be an enclosed space which can be under pressure if desired. These leaks may be generated during the actual manufacture of the apparatus, during the mounting/assembly of the apparatus and during operation of the apparatus because of stresses and corrosion.
In the previous practice of checking leak in process equipment and apparatuses the leak control has been carried out by pressure testing, conductivity measurement, gas detection and specific mass transfer measurement on the total, operable apparatus (ref.: “A study of methods evaluating the integrity of plate heat exchangers used in the diary industry”, Int. J. of Dairy Technology, Vol. 53, No. 1, February 2000).
Leak control by pressure testing is performed by applying a pressure to one side e.g. the internal side of the apparatus, following which measurement of the pressure drop will indicate a leak in the surfaces between the product and service sides.
This technology is vitiated by the general weakness that it presupposes that the internal part of the apparatus has to be sealed tightly, and a measurable pressure drop within a reasonable measuring period requires a considerable leak between the internal and external side.
Conductivity measurement for leak control is based on the principle that if an electrolyte is added to e.g. the internal side of a water-filled apparatus, transfer of this electrolyte via leaks in the surfaces will result in an increase in the conductivity in the liquid present on the external side of the apparatus. Leak determination of this type is normally performed by operating pressure on the electrolyte side and circulating an amount of water across a conductivity meter on the other side.
A serious drawback of this method of leak determination is that it requires a considerable electrolyte transfer before a significant measurement of conductivity can be achieved. By way of example it may be mentioned that it requires a transferred amount of a 6 w/V % Na2SO4 solution of about 20 ml per 100 liters of circulating recipient water in which the conductivity is measured, to achieve a significant measurable change in conductivity of the order of 10 μS, which is the value used by Bactoforce as a lower limit of leak determination.
The gas detection method which is described in U.S. Pat. No. 4,688,627 is based on the principle that a pressurized gas (e.g. helium) is applied to one side of an apparatus, following which the occurrence of this gas is measured by a gas detector on the other side.
This method is vitiated by the basic drawback that the apparatus must be completely drained and entirely dry, which is impossible to achieve in practice.
A large number of other methods based on the specific mass transfer method rely on the simple principle that a specific measurable substance is applied to the internal side or the external side, while a clear liquid is applied to the other side, said clear liquid being recirculated so that the presence of leaks in the apparatus is observed by detecting the presence of the specific substance in the clear liquid as described in inter alia U.S. Pat. No. 5,759,857; U.S. Pat. No. 5,170,840; EP No. 0 597 659 A2; U.S. Pat. No. 4,328,700; and U.S. Pat. No. 3,790,345.
It is common to all mass transfer methods that they are vitiated by the basic problem that detection of leaks requires a considerable mass transfer, and that the sensitivity is inversely proportional to the size of the apparatus.
The technology currently known and used for the control of leaks in an apparatus between the internal and the external side with a view of controlling leaky elements is not useful for in situ leak control—independently of the selected technique or combination of known techniques.
In addition, the technologies are associated with very serious drawbacks and costs which make them unuseful in general.
The most serious drawback of the known technology for leak determination and localisation of leaks in process apparatuses is that the sensitivity of the measuring methods used for leak control is inversely proportional to the size of the apparatuses.
Examples of other serious drawbacks are that the technology is deficient or not suitable with respect to achieve a leak control which is reliable and complete under operational conditions, that the technology is not sufficiently sensitive to demonstrate small leaks in process apparatuses, and that the technology is very costly and time-consuming.
Thus, there is great need for new technologies by means of which it may be ensured quickly, uniformly and in a completely reproducible manner in situ and with exact limitations of operating conditions that the apparatus operate optimally, in that the surface between the internal and the external side of the apparatus is completely tight without any risk of transfer and cross-contamination of the liquid substances which are treated in the apparatus.
To be able to measure and document the purity of the product side in apparatuses and process systems which are cleaned by CIP (“cleaning-in-place”) cleaning, the effectiveness of the cleaning performed, and the ease of cleaning of the individual parts of the apparatus, a large number of methods have been developed which are within the fields of gravimetrical, physico-chemical, microbiological and physical methods (Fat. Sci. Technol., December 1989, p. 621-624).
It is moreover known from Danish patent No. 155 627 to detect contamination in closed process systems by measuring the conductivity of the cleaning liquid.
It is common to all these methods that none of the methods alone give any qualified picture of the purity of the total production surfaces in a closed process system in terms of a distinct evaluation of the risk of contamination o the next production.
For the control of the purity of tableware (saucers, cups, etc.) with respect to residues and coatings of protein and starch, Nordic Food Products Methology Committee (no. 4, vol. 2, 1962) has described a chemical examination method which, by means of a chemical reaction between protein and starch and fuchsin and iodide/potassium iodide, respectively, is capable of showing and thereby visually the presence of protein and starch, respectively, on tableware (saucers, cups, etc.).
The principle of this method—to demonstrate the presence of organic residues on surfaces by a chemical reaction between the organic residues and a specific chemical tracer, following which the reduction of the tracer added to the circulating flushing water is measured—is described as a qualitative and quantitative method for purity control of closed process systems in Danish Published Application No. 0992/92.
This method is vitiated by the quite general weakness that the water volume necessary for circulation in a closed process system is generally very large compared to the degree of contamination of the internal surfaces of the system, so that a measurable change in the concentration of an added tracer to this water volume either requires a considerable chemical reaction between tracer and contaminant or a measuring device capable of recording even very small changes in concentration. This means that either the measuring period, during which the chemical reaction proceeds, must be very considerable, or that the measuring device used must be extremely sensitive. This means in both cases that the use of the method is very limited, and that the method is not suitable for on-line/in-line systems. Finally, this form of methods for purity control of closed process systems means that there is an undesirable relation between sensitivity and amount of water.
The technology currently known and available for purity control of the internal surfaces of closed process systems with a view to controlling and optimising the hygiene—independently of the selected technique or combination of known techniques—is not suitable for in situ purity control of process apparatuses with such a great and size-independent sensitivity as is required. Moreover, the technologies involve very considerable drawbacks and costs which cause them to be unuseful in general.
Thus, there is a great need for new technologies which can ensure quick, uniform purity control and in a completely reproducible manner in situ and with exact limitations of operating conditions wherein the process apparatus operates optimally when the internal surfaces or process surface are totally pure, so that a product contacting the surfaces is not contaminated by contaminants from the preceding production.